[RFC] Request API and V4L2 capabilities

Hans Verkuil
2018-08-04T15:50:56Z

Hi all,

While the Request API patch series addresses all the core API issues, there
are some high-level considerations as well:

1) How can the application tell that the Request API is supported and for
which buffer types (capture/output) and pixel formats?

2) How can the application tell if the Request API is required as opposed to being
optional?

3) Some controls may be required in each request, how to let userspace know this?
Is it even necessary to inform userspace?

4) (For bonus points): How to let the application know which streaming I/O modes
are available? That's never been possible before, but it would be very nice
indeed if that's made explicit.

Since the Request API associates data with frame buffers it makes sense to expose
this as a new capability field in struct v4l2_requestbuffers and struct v4l2_create_buffers.

The first struct has 2 reserved fields, the second has 8, so it's not a problem to
take one for a capability field. Both structs also have a buffer type, so we know
if this is requested for a capture or output buffer type. The pixel format is known
in the driver, so HAS/REQUIRES_REQUESTS can be set based on that. I doubt we'll have
drivers where the request caps would actually depend on the pixel format, but it
theoretically possible. For both ioctls you can call them with count=0 at the start
of the application. REQBUFS has of course the side-effect of deleting all buffers,
but at the start of your application you don't have any yet. CREATE_BUFS has no
side-effects.

I propose adding these capabilities:

#define V4L2_BUF_CAP_HAS_REQUESTS 0x00000001
#define V4L2_BUF_CAP_REQUIRES_REQUESTS 0x00000002
#define V4L2_BUF_CAP_HAS_MMAP 0x00000100
#define V4L2_BUF_CAP_HAS_USERPTR 0x00000200
#define V4L2_BUF_CAP_HAS_DMABUF 0x00000400

If REQUIRES_REQUESTS is set, then HAS_REQUESTS is also set.

At this time I think that REQUIRES_REQUESTS would only need to be set for the
output queue of stateless codecs.

If capabilities is 0, then it's from an old kernel and all you know is that
requests are certainly not supported, and that MMAP is supported. Whether USERPTR
or DMABUF are supported isn't known in that case (just try it :-) ).

Strictly speaking we do not need these HAS_MMAP/USERPTR/DMABUF caps, but it is very
easy to add if we create a new capability field anyway, and it has always annoyed
the hell out of me that we didn't have a good way to let userspace know what
streaming I/O modes we support. And with vb2 it's easy to implement.

Regarding point 3: I think this should be documented next to the pixel format. I.e.
the MPEG-2 Slice format used by the stateless cedrus codec requires the request API
and that two MPEG-2 controls (slice params and quantization matrices) must be present
in each request.

I am not sure a control flag (e.g. V4L2_CTRL_FLAG_REQUIRED_IN_REQ) is needed here.
It's really implied by the fact that you use a stateless codec. It doesn't help
generic applications like v4l2-ctl or qv4l2 either since in order to support
stateless codecs they will have to know about the details of these controls anyway.

So I am inclined to say that it is not necessary to expose this information in
the API, but it has to be documented together with the pixel format documentation.

Comments? Ideas?

Regards,

Hans